Process for the recovery of high-boiling ingredients from coal tar by distillation



PROCESS FCR THE RECQJVERY F lHGH-BOILING ggREDENTS FROM COAL TAR BYDISTHJLA- Julius Geller and Richard Schumacher, Bad i-lomburg vor derHohe, and Karl Friedrich Lang, Frankfurt am Main, Germany; said Gellerand said Schumacher assiguors to 'Riitgerswerke-Alrtiengesellschaft,Frankfurt am Main, Germany Applicafion August 27, 1956, Serial No.606,240

Claims priority, application Germany August 29, 1955 I 7 Claims. (Cl.208-366) This invention relates to the continuous production bydistillation of high-boiling ingredients of coal tar, which becomehighly concentrated in highly-boiling coal tar fractions, in order tofacilitate further processing, for example, the recovery of pureproducts such as anthracene, carbozol or pyrene, and to increase theyields of these ingredients.

In the processes known from the prior art, the unlimited application ofrectification for the separation and recovery by distillation of certaincoal tar ingredients, some of which are very valuable, was prevented bythe lack of thermal stability of said coal tar ingredients.

In view of the above reason, processing by distillation of the coal tarby evaporation processes in one or more steps, has retained its economicimportance for a long time, although in the application of theseevaporation processes, the yield and concentration of certain iugredients which are of economic importance, are relatively low so thatfurther processing is rather expensive. In this connection the followingevaporation processes known from the art can be mentioned.

One-step evaporation (1) The coal tar charge of a single evaporator issubjected to evaporation without reflux and the condensed products arerecovered in different receivers (discontinuous distillation).

(2) The coal tar is continuously introduced into an evaporator and issubjected to evaporation without refiux up to about one-half of thecharge in the evaporator. The vapors are introduced in succession intocondensers which are cooled to varying degrees and are thus subdividedby fractional condensation.

Evaporation in several steps (3) The coal tar is continuously passedsuccessively through several evaporators which are heated to increasingtemperatures. The fractions of the individual evaporation steps arecondensed and collected after condensation in separate receivers.

(4) The coal tar is continuously passed at a constant temperaturesuccessively through evaporators which are kept under increasing vacuum.The vapors of the individual vacuum steps are thus separated accordingto their boiling ranges. (5) It has also been known from the art tosubject individual coal tar fractions resulting from one of the abovementioned evaporation processes, or a combination thereof, to furtherprocessing by rectifying redistillation.

In the case of the above mentioned evaporation process (1), this can bedone discontinuously only, while in the case of processes (2)(4), it canbe done discontinuously or continuously. A process corresponding to theabove mentioned procedure (4) is, for example, described in GermanPatent No. 260,060. A process 2,03,50 Patented July i, 1059corresponding to a combination of the above sections (4) and (5) hasalso been known from the art.

A known process based on the combination of the procedures according tothe above sections (2), (3) and (4) is described in US. Patent No.2,029,883. The process described in German Patent No. 767,001 utilizesthe basic process mentioned above in section (2) and combines it withredistillation according to the above section (5), whereby the surfaceevaporator according to German Patent No. 260,060 is replaced by a tubefurnace, like in US. Patent No. 2,020,883.

In all of the above mentioned processes for the treatment of coal tar bydistillation, the crude tar is supplied only with the amount of heatwhich is sufficient for a single evaporation of the tar fractions to beseparated therefrom. The yields and concentrations of the economicallymost important tar ingredients in the thus separated fractions areunsatisfactory and cause, therefore, relatively high expenses resultingfrom the necessary subsequent treatment.

The yields and concentrations can be increased by discontinuousrectification of the coal tar with heat supply to the coal tar still,but the discontinuous character of the procedure and the thermalinstability of the pitch remaining in the still, limit said increase ofyields and concentrations.

French Patent No. 1,018,333 describes a process for the recovery ofcarbozol and an anthracene-phenanthrene mixture by continuousdistillation of coal tar. In this process, heated coal tar which hasbeen freed from the ingredients boiling below phenanthrene, is subjectedto distillation in an evaporator provided with a column in order todistill ofii-in addition to higher boiling ingredientsfirst of all, acrude carbazol fraction. The latter is separated in a fractionatingcolumn, having a stripping portion and a side column, into a headfraction containing mainly anthracene and phenanthracene, into one ormore intermediate fractions and a sump product which contains mainlycarbazol.

In this last mentioned process, too, the object of which is the recoveryof high-boiling ingredients of coal tar, it has still been avoided tosupply to the sump product of the respective column the amount ofrectification heat, which is necessary for a complete sharp separationand high yield in separating the carbazol fraction. It is for thisreason that the pitch discharged from the column sump still containscarbazol and pyrene in this process.

It has now been found that the difficulties and disadvantages of theknown processes can be avoided by proceeding according to the process ofthis invention as follows. v

In this process, all coal tar ingredients boiling below 300 C. areseparated under vacuum. Subsequently the heavy fractions of coal tarhaving a boiling range above 300 C. up to about 400 C. are recovered ina fractionating column (pitch column) from the residual coal tar bytransmitting the rectification heat to the sump product of this column,in a fraction denoted herein collective fraction, as the head product ofsaid column and said head product is subsequently separated in 3 or morefractionating columns connected in series, into the desired heavyfractions.

The new process of this invention avoids the disadvantages of the priorart processes and yields highly concentrated fractions with high yields,which can be further processed to pure products in a simple manner.

According to the invention, in the fractionating columns arranged insuccession and connected in series, first a phenanthrene-anthracenefraction, subsequently an acridine fraction, and finally a carbazolfraction, is separated, whereby a fluoranthene-pyrene fraction isrecovered as the sump product of the last fractionating column.

The fluoranthene-pyrene fraction, which is the highest boiling andlargest heavy fraction obtained from the head product of the pitchcolumn, is used as the carrier of rectification heat in all distillationsteps after the pitch column.

The present invention is based on novel and fundamental considerationsand tests, the results of which could not be expected and aresurprising. This invention is the result of a combination of inventiveconcepts which are not in harmony with the conventional ideas andexperiences in connection with the processing by distillation of coaltar ingredients boiling in the range of about 300400 C.

It has been unexpectedly found that it is entirely possible to supplyconsiderable amounts of heat to a briquetting pitch having asolidification point of about 75 -85 C., without adversely affecting thequality of the pitch in the manner assumed in the hitherto prevailingviews of the art, if by suitable selection ofthe individual fractionsand sharpness of separation in the pitch column in the manner describedhereinafter, care is taken to avoid the presence in the sump of thepitch column of ingredients which are liable to decomposition and ofcoal tar ingredients which cause the occurrence of resinification andcoke formation, and boil below approximately 385 C.

Comparative tests have indicated that in the thermal treatment of coaltar ingredients boiling above about 300 C. and up to 400 C., first ofall, the linear B-nuclear substances of the coal tar, for example:anthracene, acridine and carbazol, are considerably liable to decomposition, particularly in the vapor phase. Thereby, on the one hand, thecarbon content of the pitch is increased, and, on the other hand, theyield of valuable ingredients of the coal tar is reduced in anundesirable manner.

Such undesirable decomposition is reduced, or even extensivelyprevented, by the following basic process steps:

(a) In the first distillation step, in contrast to the French Patent No.1,018,333, the charged material which still contains anthracene andcarbazol, is not heated under a predetermined pressure in a tubefurnace, but an additional amount of rectification heat is transmittedonly to the circulating sump product of the pitch column, which is freefrom anthracene and carbazol.

(b) By simultaneous separation of the fluoranthenepyrene fraction fromthe residual coal tar, the anthracene and carbazol concentration of thecollective fraction is reduced and thereby decomposition of the linear3-nuclear substances in the vapor phase is prevented.

The fact that the collective fraction contains the fluoranthene-pyrenefraction as a diluting medium has an advantageous effect, because saiddiluent reduces the solidify ng point of this fraction to a considerableextent, so that the first column can be operated under a considerablyhigher vacuum than the three subsequent columns connected therewith.Although the pressure in the first column could be reduced up toabout 1Torr. in order to avoid decomposition, it has been found that it iseconomically suificient to adjust said pressure to about 20 to 50 Torr.,While the pressure in the subsequent columns must normally amount toabout 100 to 300 Torr.

It has been found that it is of extraordinary advantage to have in thesubsequent distillation steps for the separation of the anthracenefraction, recovery of an acridinecontaining intermediate fraction andfirst of all for the distillation of the carbazol fraction, a thermallymore stable butter available dueto the presence of thefluoranthene-pyrenc fraction, which is passed through the sumps of thesecolumns and renders it possible to supply the rectification heatnecessary for a satisfactory separation of the most important compounds,to these distillation systems arranged after the pitch column.

In order to further reduce the carbazol concentrations and thus restrainthe tendency of decomposition in the sumps of the subsequent columnsfollowing the pitch column, it has been further found to be of advantageto '4 t admix part of the fiuoranthene-pyrene fraction in a cycle to thehead product of the first fractionating column (2).

In order to restrain the tendency of decomposition of the 3-nuclearsubstances, particularly carbazol, which are present in increasedconcentrations in the evaporating parts of said subsequent columns,steam can be introduced, if desired, according to the invention into thecolumns or into the heaters for the sump product; This steam, like thefluoranthene-pyrene fraction present in the first step of the process,has an effect of reducing decomposition in the respective steps,particularly in the recovery of carbazol.

By the above mentioned steps of the invention, it is attained thatanthracene, acridine as well as carbazol, appear in concentrated formpractically only as distillates and need not be taken up as sumpproducts by correspoudingly higher temperatures, so that, for example inthe case of carbazol, the yield is increased by more than 60% incomparison with the procedure in which carbazol is obtained as a sumpproduct.

The process of the invention is diagrammatically illustrated in theappended drawing.

A particular feature of the process consists in that part of thefluoranthene-pyrene fraction is used, after cooling to about 120 C., asan effective washing agent for the waste gases of the columns which areoperated under vacuum. This fraction is particularly suitable as awashing agent for high-boiling ingredients of coal tar, because thisfraction does not solidify when cooled even up to 20 C. and is notvolatile at the temperatures used in the process, in contrast to normalwashing oil, which is conventionally used for washing in vacuumprocesses.

The portion of the fluoranthene-pyrene fraction, which has been used asa washing agent, is then preferably added in a cycle to the distillateof the first fractionating column and introduced with this product asfeed of the column (2). In this manner, losses of material in the wastegases of the subsequent column are avoided and the yield is stillfurther increased.

Example In carrying out the invention, first the ingredients boilingunder 300 C., which amount to about 25% of the coal tar, are separatedfrom the coal tar. The residual coal tar is introduced into column 1,from which about of the charge introduced into the column are obtainedas pitch having a solidification point of about -85 C., preferably aboutC., as the sump product, while about /3 of the charge are discharged athead product of said column 1. To a portion of the sump product,rectification heat is supplied=-by means of a tube furnace 5 operatedunder adjustable pressure-for the sharp separation of pyrene from thepitch, in such an amount that the vapor condensed in condenser 9approximately corre sponds to the amount ofthe introduced product. At apressure of, for example, 30 Torr. (measured at the head of the column),the head temperature will amount to about 240 C.

The collective fraction obtained as the head product in column 1 (saidfraction corresponding to about 25% of the coal tar) contains threeheavy fractions comprising as the most important componentsphenanthreneanthracene, acridine, carbazol, methylcarbazol, fluorantheneand pyrene. This fraction contains the linear 3-ni1- clear substances(which are relatively easier decomposed than the non-linear 4nuclearingredients of the fraction), for example, anthracene, in an amount ofabout 7% and carbazol in an amount of about 8%. This corresponds toabout 1.75% of anthracene and 2.07% carbazol based on the amount of coaltar.

Anthracene is recovered, together with phenanthrene, in column 2 as thehead product in an amount corresponding to about 6.3% of the coal tar.Acridine and carbazol are discharged together with an about 3.5 timeshigher amount of a fluoranthene-pyrene fraction which acts as astabilitybuffer, as the sump productof this 001- umn, which is suppliedto a middle plate of the successiveadjacent column 3. Under a pressureof about 200 Torr, the temperature amounts, for example, at the head toabout 270 C.

The acridine fraction (which is an intermediate fraction and correspondsto about 1.7% of the coal tar) is obtained as the head product in column3, While the carbazol compounds-together with the fluoranthenepyrenefraction which is then present in an about 6.5 times higher amount andacts as a stability bufleris recovered as the sump product of thiscolumn and serves as the material to be supplied to column 4. Thetemperatures amount, for example, at the head of the column up to about280 C. at a pressure of about 200 Torr.

In column 4, the carbazol fraction (which corresponds to about 2.4% ofthe coal tar) is obtained as the head product and thefluoranthene-pyrene fraction (which corresponds to about 14.6% of thecoal tar) is discharged preferably together with the methyl carbazols asthe sump product. The temperatures at the head amount up to about 290 C.under a pressure of about 200 Torr.

As in the stripping portions and the sumps of columns 2-4 (which areconnected in succession With each other and with column 1), thethermally more stable fluoranthene-pyrene fraction is present in eachcase as a stability buifer, it is possible to supply to the sump productof columns 2-4 by means of tube furnaces 6, 7 and 8, such amounts ofrectification heat which are sufiicient to cause in condensers 10, 11and 12 reflux liquid in amounts which are 2-3 times higher than theamount charged in each case to said columns. A portion of thefluoranthene-pyrene fraction can be used in scrubber 13 for washing thegases before they enter the vacuum pump and said portion can be thenadded to the head product of column 1, whereby any loss of valuabledistillates is eliminated.

By carrying out the process of the invention in conformity with theabove example, the following results are obtained.

The carbazol obtained as the head product of column 4 shows thefollowing analysis by fractional distillation:

Start 40% 50% 60% 70% 80% 90% This carbazol is practically free frommethylcarbazols.

The head product of column 4 has a carbazol content of 36%, whichcorresponds to a yield of at least 73% based on the total amount ofcarbazol present in the coal tar.

The head product of column 2 has an anthracene content of about 25%,which corresponds to a yield of at least 88% based on the total amountof anthracene present in the coal tar. This fraction can be furtherprocessed in a separate step.

The head product of column 3 contains as an intermediate fraction-inaddition to acridine which is characteristic of this fraction--by farmost of the residual amount of anthracene and carbazol, while the sumpproduct of column 4 contains the methylcarbazols, together withfluoranthene and pyrene.

The above example illustrates the unexpectedly high yields of anthracene(about 88%) and carbazol (about 7 3%) which are obtainable by theprocess of this invention (based on the total amount of said ingredientspresent in the coal tar used as starting material).

It should be emphasized that the ratio of anthracene/ carbazol in theanthracene fraction amounts to more than 15/ 1 and in the carbazolfraction to about l/ 36. These yields and degrees of purity ofierconditions for further processing which are economically extremelyfavorable.

It will be understood from the above that the present invention 'is'based on a'novel and original combination of steps and that thiscombination yields unexpected and surprising results. In carrying outthis invention, first all ingredients which boil below 300 C. underatmospheric pressure, are removed from the coal tar by distillation. Theresulting residual coal tar, which contains the ingredients boilingabove 300 C. under atmospheric pressure, is introduced into a firstfractionating column, in which the ingredients which boil above 300 C.and up to about 400 C. and include phenanthrene, an thracene, acridine,carbazol, fluoranthene and pyrene, are vaporized to form afraction-which is denoted collective fraction herein and is recovered asthe head product in said first fractionating column, which is operatedunder vacuum of, e.g. 30 Torr. (measured at the head of the column),whereby the temperature at the column head is about 240 C. The sump ofthis first fractionating column contains pitch which is free fromanthracene and carbazol, and to which rectification heat is supplied bycausing part of it to circulate through a tube furnace. Theamount ofsaid rectification heat is selected in such a manner that the taringredients which boil in the range of 300400 C. and amount to about 25%by weight of the coal tar starting material, are vaporized and escape asthe head product of the column. Thereby, the temperature in the tubefurnace 5 of fractionating column 1 amounts to about 370-410 C. and thetemperature of the pitch in the sump of column 1 amounts to about 350380C. under the above mentioned conditions.

The head product of fractionating column 1 is condensed and subjected tofurther fractionation in subsequent columns in the above describedmanner, whereby thermal decomposition is reduced or eliminated to asatisfactory extent by the presence of the fluoranthene-pyrene fractionduring said further fractionation.

It will also be understood that this invention is not limited to thespecific steps, temperatures, pressures and other details specificallydescribed above and illustrated in the drawings and can be carriedoutwith various modifications without departing from the scope of theinvention as defined in the appended claims.

The term Torr. used herein denotes a unit of pressure equal to of thenormal atmospheric pressure.

As mentioned in the first paragraph of the above example, the amount ofvapor condensed in condenser 9 approximately corresponds to the amountof the introduced product. Thus, for example 21 tons per hour of thestarting material are introduced into the middle of column 1. About /3of this amount, i.e., 7 tons per hour, are vaporized and then condensedin condenser 9 and passed as distillate to column 2. However, in tubefurnace 5 additionally an amount of heat is produced, which issufiicient for vaporizing 14 tons per hour. These 14 tons represent theinner circulation of vaporized liquid of the column, which isadditionally condensed in condenser 9 and flows from there to the headof the column 1, in which it is vaporized again.

What is claimed is:

1. A process for the recovery of high-boiling ingredients from coal tarby distillation, comprising (at) removing by distillation from coal tarthe ingredients having under atmospheric pressure boiling points below300 C.; subjecting the residual tar resulting from step (a) to a firstfractional distillation under vacuum in order to separate said residualtar into a vaporized collective fraction containing ingredients havingunder atmospheric pressure boiling points in the range of 300-400 C. anda pitch residue which has a solidification point of 75 -85 C. and issubstantially free from anthracene and carbazol, heat being supplied tosaid pitch residue only by causing part of it to circulate through anelongated heating zone of restricted cross-section at a temperature inthe range of 3704l0 C., whereby the pitch residue in the sump is heatedto about 350 380 C.; condensing the collective fraction and separatingthe ingredients thereof by subsequent fractional vacuum distillationsconnected with each other in series.

2. A process as claimed in claim 1, in which the collective fraction isseparated in succession into: (a) a phenanthrene-anthracene fraction;(b) an acridine fraction; (c) a carbazol fraction, and (d) afluoranthrene-pyrene fraction, said fractions (a), (b), and (c), beingobtained as head products, and said fraction (d) being obtained as thesump product of the last fractional distillation; the necessaryfractionation heat being supplied to these fractional distillations bycausing parts of the sump products to circulate through a tube furnace.H

3. A process as claimed in claim 2, in which part of thefluoranthrene-pyrene fraction is added in a cycle to the head product ofthe first fractionation of the collective fraction. Y

4. A process as claimed in claim 2, in which steam is introduced intosump products resulting from the fractionation steps. V

5. A process as claimed in claim 2, in which part of thefluoranthene-pyrene fraction is cooled to ahoi1t120 C. and used as awashing fluid for the Waste gases of vacuum fractionations;

6. A process as claimed in claim 5, in which used washing fluid is addedin acycle to the head product of the first fractionation of thecollective fraction.

7.- A process as claimed in claim 1, in which vaporization oftheingredients having boiling points in the range of 300-400 C. is carriedout under a higher vacuum than the subsequent fractional vacuumdistillations of the collective fraction.

References Cited in the file of this patent UNITED STATES PATENTS2,029,833 MacC'ubBin er a1. Feb. 4, 1936 2,099,434 Culbertson Nov. 16,1937 2,203,930 Smith J ne 11, 1940 2,250,072 Wilton Oct. 21, 19412,594,352 Schmalenbach Apr. 29, 1952 FOREIGN PATENTS 303,038 GreatBritain Dec. 27, 1928

1. A PROCESS FOR THE RECOVERY OF HIGH-BOILING INGREDIENTS FROM COAL TARBY DISTILLATION, COMPRISING (A) REMOVING BY DISTILLATION FROM COAL TARTHE INGREDIENTS HAVING UNDER ATMOSPHERIC PRESSURE BOILING POINTS BELOW300*C., SUBJECTING THE RESIDUAL TAR RESULTING FROM STEP (A) TO A FIRSTFRACTIONAL DISTILLATION UNDER VACUUM IN ORDER TO SEPARATE SAID RESIDUALTAR INTO A VAPORIZED COLLECTIVE FRACTION CONTAINING INGREDIENTS HAVINGUNDER ATMOSPHERIC PRESSURE BOILING POINTS IN THE RANGE OF 300*-400*C.AND A PITCH RESIDUE WHICH HAS A SOLIDIFICATION POINT OF 75*-85*C. AND ISSUBSTANTIALLY FREE FROM ANTHRACENE AND CARBAZOL, HEAT BEING SUPPLIED TOSAID PITCH RESIDUE ONLY BY CAUSING PART OF IT TO CIRCULATE THROUGH ANELONGATED HEATING ZONE OF RESTRICTED CROSS-SECTION AT A TEMPERATURE INTHE RANGE OF 370*-410*C., WHEREBY THE PITCH RESIDUE IN THE SUMP ISHEATED TO ABOUT 350*-380*C., CONDENSING THE COLLECTIVE FRACTION ANDSEPARATING THE INGREDIENTS THEREOF BY SUBSEQUENT FRACTIONAL VACUUMDISTILLATIONS CONNCETED WITH EACH OTHER IN SERIES.